The present invention relates to a Raman amplified optical communication system and, more particularly, to an optical communication system utilizing co-propagating Raman amplification with Raman pump sources particularly designed to overcome known pump-signal crosstalk problems.
The subject of Raman amplification is well known in the literature. Stimulated Raman amplification is a nonlinear optical process in which an intense pump wave is injected into an optical fiber that is carrying one or more optical signals. In fused silica fibers, if the pump wave is of a frequency approximately 13 THz greater than the signal waves (i.e., if the pump wavelength is approximately 100 nm shorter than the signal wavelength in the vicinity of 1500 nm), the pump will amplify the signal(s) via stimulated Raman scattering. If the amplification is made to occur in the transmission fiber itself, the amplifier is referred to as a xe2x80x9cdistributed amplifierxe2x80x9d. Such distributed amplification has been found to improve the performance of a communication system, as discussed in the article xe2x80x9cCapacity upgrades of transmission systems by Raman amplificationxe2x80x9d by P. Hansen et al. appearing in IEEE Phot. Tech. Lett., Vol. 9, 1997, at page 262. For example, if a pump wave is injected into one end of the fiber in a direction that is counter-propagating with respect to the information signals, the signals will be amplified before their signal-to-noise ratio degrades to an unacceptable level. The performance of such an amplifier is often characterized in terms of its xe2x80x9ceffectivexe2x80x9d or xe2x80x9cequivalentxe2x80x9d noise figure and its on/off gain. The effective noise figure is defined as the noise figure that an equivalent post-amplifier would have in order to achieve the same noise performance as the distributed Raman amplifier (see, for example, xe2x80x9cRayleigh scattering limitations in distributed Raman pre-amplifiersxe2x80x9d, by P. Hansen et al., IEEE Phot. Tech. Lett. Vol. 10, 1998, at page 159). Experimentally, the effective noise figure may be found by measuring the noise figure of a span utilizing counter-propagating Raman amplification and then subtracting (in decibels) the passive loss of the span. The on/off gain of a distributed Raman amplifier is defined as the difference (in decibels) between the output signal power with the Raman pump xe2x80x9conxe2x80x9d to that with the pump xe2x80x9coffxe2x80x9d. Alternatively, a lumped or xe2x80x9cdiscretexe2x80x9d amplifier can be constructed with a local length of Raman gain fiber.
It is well known in the prior art that Raman gain generated with a polarized pump wave is, in general, polarization dependent. This phenomenon is discussed in detail in an article entitled xe2x80x9cPolarization effects in fiber Raman and Brilloiun lasersxe2x80x9d by R. H. Stolen et al., appearing in IEEE J. Quantum Electronics, Vol. QE-15, 1979, at p. 1157. Given that the vast majority of fiber optic communication systems utilize non-polarization maintaining fibers, an optical signal""s state of polarization at any given point is not generally known and is subject to capricious variations. For these reasons, it is desirable to minimize polarization-dependent loss and gain within the communication system. It has also been shown that the polarization dependence of Raman amplifiers can be significantly reduced by polarization multiplexing polarized Raman sources, as disclosed in U.S. Pat. No. 4,881,790, issued to L. F. Mollenauer et al. on Nov. 21, 1989.
Significant pump powers are required to generate substantial on/off Raman gain in conventional transmission fibers. For example, approximately 300 mW of power is required from a monochromatic pump to generate 15 dB of on/off Raman gain in transmission fibers with xcx9c55 xcexcm2 effective areas. It is also known that these pump powers are significantly higher than the threshold for stimulated Brilloiun scattering (SBS) for pump sources with spectral widths less than 25 MHz, as discussed in the article xe2x80x9cOptical Power Handling Capacity of Low Loss Optical Fibers as determined by Stimulated Raman and Brilloiun Scatteringxe2x80x9d, by R. G. Smith, appearing in Appl. Optics, Vol. 11, 1972, at page 2489. Stimulated Brilloiun scattering is a well-known nonlinear optical process in which the pump light couples to an acoustic wave and is retro-reflected. This retro-reflection may prohibit the penetration of the Raman pump significantly deep into the transmission fiber, inhibiting the generation of Raman gain.
The threshold for SBS can be substantially increased by broadening the spectral width of the Raman pump source, as discussed in the above-cited Mollenauer et al. patent. In particular, one method for broadening the spectral width and thus suppressing SBS is by frequency dithering of the laser source. Another mechanism for broadening the spectral width of a laser is to allow the device to lase in more than one longitudinal mode of the laser cavity. The frequency spacing of the longitudinal modes of a laser is defined by the relation c/2 ngL, where c is the speed of light in a vacuum, ng is the group velocity within the laser cavity and L is the length of the cavity.
Certain types of semiconductor lasers are preferred for use as Raman pump sources. The most common types of semiconductor pump lasers are Fabry-Perot (FP) lasers, and FP lasers locked to external fiber Bragg gratings. These types of pump sources are discussed in an article entitled xe2x80x9cBroadband lossless DCF using Raman amplification pumped by multichannel WDM laser diodesxe2x80x9d by Emori et al. appearing in Elec. Lett, Vo. 34, 1998 at p. 2145. It is typical for the external fiber Bragg gratings to be located approximately 1 m from the semiconductor laser.
It is known that when light from a laser, lasing in multiple longitudinal modes, is passed through a dispersive delay line (such as an optical fiber), noise components referred to as mode partitioning noise are generated at frequencies typically less than a few GHz. See, for example, xe2x80x9cLaser Mode Partitioning Noise in Lightwave Systems Using Dispersive Optical Fiberxe2x80x9d, by R. Wentworth et al., appearing in J. of Lightwave Technology, Vol. 10, No. 1, 1992 at pp. 84-89. It is also known that single-longitudinal-mode semiconductor lasers are typically used as signal sources. Common types are distributed feedback (DFB) lasers and distributed Bragg reflector (DBR) lasers.
The Raman amplification process is known as an extremely fast nonlinear optical process. For this reason, intensity fluctuations in the pump may result in fluctuations in the Raman gain. These gain fluctuations may then impress noise upon the optical signals, degrading the performance of the communication system. For the purposes of understanding the teaching of the present invention, this effect will be referred to as the xe2x80x9cpump-signal crosstalkxe2x80x9d. It is known that, at sufficiently high frequencies, the signal and pump will xe2x80x9cwalk offxe2x80x9d with respect to one another, due to dispersion within the fiber. It is also known that the use of a strictly counter-propagating pump geometry, that is, where the direction of propagation of all Raman pumps is opposite to that of all signals, is effective in reducing degradations from pump-signal crosstalk. This amplifier geometry is discussed in detail in an article entitled xe2x80x9cProperties of Fiber Raman Amplifiers and their Applicability to Digital Optical Communication Systemsxe2x80x9d by Y. Aoki, appearing in J. Lightwave Technology, Vol. 6, No. 7, 1988 at pages 1225-29. In counter-propagating pump geometries, the transit time through the amplifying fiber is used to average the pump intensity fluctuations such that xe2x80x9cquietxe2x80x9d amplification may be achieved. It is also known that the counter-propagating pump geometries serve to reduce the polarization dependence of the Raman gain.
Another potential source of noise in Raman amplified systems arises in systems transmitting information in multiple signal wavelengths, where the multiple signals will more quickly deplete the power in the Raman pump. See, for example, xe2x80x9cCrosstalk in Fiber Raman Amplification for WDM Systemsxe2x80x9d, W. Jiang et al., J. of Lightwave Technology, Vol. 7, No. 9, 1989 at pp. 1407-111. In this situation, the information imposed on one signal wavelength is impressed upon a signal at the same or a different wavelength via the Raman gain process. For the purposes of understanding the teaching of the present invention, this effect will be referred to as the xe2x80x9csignal-pump-signal crosstalkxe2x80x9d. This source of noise is also greatly reduced in counter-propagating pump geometries where the transit time through the amplifying fiber is used to reduce the effects of any pump intensity fluctuations.
It is also known that due to unusual noise sources, such as pump-signal crosstalk and signal-pump-signal crosstalk, it is often necessary to characterize the noise performance of Raman amplifiers with electrical noise figure measurements, characterizing the effective noise figure as a function of electrical frequency.
There are potential system advantages to the use of co-propagating Raman amplification, including increasing the signal-to-noise ratios of the amplified signals, minimizing excursions of the signal powers as a function of length, and allowing for the bi-directional propagation of signals within the same distributed Raman amplifier. However, a problem with these co-propagating Raman amplifiers is that they are more susceptible to both pump-signal crosstalk and signal-pump-signal crosstalk.
An exemplary prior art co-propagating Raman amplifier arrangement is discussed in the article xe2x80x9cWide-Bandwidth and Long-Distance WDM Transmission using Highly Gain Flattened Hybrid Amplifiersxe2x80x9d by S. Kawai et al., appearing in IEEE Phot. Tech. Lett., Vol. 11, No. 7, 1999 at pp. 886-888. However, the on/off Raman gain of this particular configuration is exceedingly low (i.e., approximately 4 dB)xe2x80x94a region where the above-mentioned problems would be minimal.
Thus, a need remains for a co-propagation Raman amplification system that provides a sufficient on/off gain to be a useful device, while not exhibiting undesirable levels of pump-signal crosstalk and signal-pump-signal crosstalk.
The need remaining in the prior art is addressed by the present invention, which relates to Raman amplified optical communication system and, more particularly to an optical communication system utilizing co-propagating Raman amplification with Raman pump sources particularly designed to overcome pump-signal crosstalk problems in co-propagating systems.
In accordance with the present invention, an optimized Raman pump source is utilized that produces at least 50 mW of output power, sufficient spectral width to suppress SBS, and is configured such that the frequency difference between all intense longitudinal pump modes (regardless of polarization) are separated by at least the electrical bandwidth of the communication system, or at least the walk-off frequency, where xe2x80x9cwalk-off frequencyxe2x80x9d is defined as the lowest frequency at which the pump-signal crosstalk is no longer a significant factor in degrading the performance of the Raman amplifier.
In various embodiments, the pump source may comprise one or more frequency-dithered DFB lasers, multi-longitudinal mode DFB lasers, DBR lasers, frequency-offset FP lasers, or FP lasers locked to a Fabry-Perot fiber Bragg grating reflector.
In one embodiment of the present invention, the pump source may be injected into the input of dispersion-compensating fiber at the input of a discrete Raman amplifier to generate co-propagating Raman amplification, where the effects of both pump-signal crosstalk and signal-pump-signal crosstalk are minimized.
Advantageously, the pump sources of the present invention may be used in either a distributed Raman amplifier application or a discrete Raman amplifier application.
Other and further embodiments of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.